During cold engine operation, including cold engine starts and shorter drive cycles in colder climates, an engine may not reach stable engine operating conditions leading to fuel dilution in engine oil. For example, fuel may mix with engine oil in an engine's crankcase. Repeated cold start engine operation without completing engine warm-up may result in excessive fuel dilution and degradation of oil quality. Further, this can cause oil overfill conditions which in turn may generate fuel odors perceptible to a vehicle operator. Excessive fuel in the oil may also decrease engine durability and affect engine performance.
In general, as engine temperature increases, fuel diluted in engine oil may be burned off. An example of the above approach is shown by Haladyna et al. in US 2014/0058647. The example approach includes starting and operating an engine in a hybrid vehicle when engine oil dilution is determined to be more than a threshold amount. Further, the engine may be operated for a given duration until a target engine temperature is attained promoting evaporation of fuel from engine oil.
However, the inventors herein have identified potential issues with the above example approach. As an example, vehicles (whether hybrid or not) may not experience long enough drive cycles for engine temperatures to warm up to the target temperature enabling fuel evaporation. Further, the engine may have to be operated for longer durations in colder ambient conditions to reach the target temperature leading to a significant increase in fuel consumption. Consequently, fuel economy of the vehicle may be considerably decreased.
The inventors herein have recognized the above issues and identified an approach to at least partly address the above issues. In one example approach, a method for a boosted engine comprises, when engine oil dilution is higher than a first threshold and engine oil temperature is below a temperature threshold, adjusting a position of an air induction system (AIS) throttle, generating a vacuum, and applying the vacuum to a crankcase to enhance fuel evaporation from engine oil. In this way, engine oil dilution may be lowered even during shorter drive cycles.
For example, when a boosted engine is cold started, oil dilution in a crankcase of the boosted engine may be determined to be higher than a first threshold. Further, evaporation of fuel from engine oil may be inhibited at the cold start as engine oil temperature may be below a temperature threshold. As such, the temperature threshold may be a boiling point of fuel diluted in the engine oil. An air induction system (AIS) throttle positioned in an intake passage upstream of a compressor in the boosted engine may be adjusted to a more closed position when engine oil dilution is greater than the first threshold and engine oil temperature is below the temperature threshold. The adjustment of the AIS throttle may be based on engine operating conditions. As an example, the AIS throttle may be closed only if a desired engine performance is met. For example, if engine torque demand is increasing, the adjustment to the AIS throttle may not be performed. By adjusting the AIS throttle to the more closed position, vacuum may be generated in the intake passage and this vacuum may be provided to the crankcase. As a result, pressure within the crankcase may be reduced enabling vaporization of fuel dissolved in the engine oil. If engine oil temperature increases beyond the temperature threshold, the AIS throttle may be adjusted to a more open position. Further, if engine oil dilution decreases below the first threshold, the AIS throttle may be opened and provision of vacuum to the crankcase may be stopped. Further still, if pressure within the crankcase reduces below a pressure threshold, the AIS throttle may be adjusted to a more open position and pressure within the crankcase may be increased.
In this way, engine oil dilution may be decreased without relying on increasing the engine oil temperature. By reducing pressure within the crankcase, a boiling point of fuel may be decreased accelerating the evaporation of fuel from engine oil. Further, vacuum generated by closing the AIS throttle may advantageously purge a headspace in the crankcase of evaporated fuel vapors. By allowing an adjustment to the AIS throttle only when engine operating conditions permit a reduced airflow, engine performance may be maintained. Overall, durability of the engine may be improved without degrading fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.